An A/D converter circuit of the time A/D (TAD) system provided with a pulse circulation circuit is proposed. The pulse circulation circuit is configured with delay units, which are coupled together in a ring shape, and which output an input pulse signal after delaying it by a delay time corresponding to a power supply voltage. This A/D converter circuit is supplied with an analog input voltage to be A/D-converted as a power supply voltage of the delay units, counts the circulation number (number of circulation) of a pulse signal in the pulse circulation circuit, and produces A/D conversion data based on the count value. The A/D converter circuit of a TAD system can be configured with digital circuit elements, such as a gate, and possesses many advantages that the circuit configuration is comparatively simple and can be realized at low cost.
However, since the delay unit (for example, inverter) with a delay time, which depends on the power supply voltage, is formed of semiconductors such as a MOS transistor, the A/D converter circuit of a TAD system has the characteristics that the delay time thereof changes depending on temperature. That is, although the power supply voltage (analog input voltage) is constant, at low temperature, the delay time of the delay unit becomes short and the circulation number to be counted increases. On the contrary, at high temperature, the delay time of the delay unit becomes long and the circulation number to be counted decreases. Accordingly, A/D conversion data changes depending on ambient temperature of the A/D converter circuit.
A compensation technology to the temperature change is disclosed by the following patent documents 1, 2, and 3, for example.
(Patent document 1) JP 4396063A (FIG. 5); US 2003/0011502A1,
(Patent document 2) JP 2008-312185A (FIGS. 12 to 16); US 2008/0309542A1
(Patent document 3) JP 2007-104475A (FIG. 13); US 2007/0080844A1
In an A/D converter circuit disclosed by patent document 1, an analog input voltage is inputted to a first pulse circulation circuit and a fixed reference voltage is inputted to a second pulse circulation circuit, then A/D conversion data is produced by numeric conversion of a ratio of transmission speed of a pulse signal produced in each pulse circulation circuit. However, accurate compensation can be attained only in the case where the analog input voltage is comparatively high, for example, higher than 5V. Therefore, due to the circumstances, it is hard to adopt the A/D converter to equipment, such as a vehicle sensor which operates with a power supply voltage of 5V.
On the other hand, an A/D converter circuit disclosed by patent document 2 allows an accurate compensation in the case where the analog input voltage is comparatively low, for example, about 2V. This A/D converter circuit utilizes a point γ (gamma), at which the difference of temperature characteristics of a delay unit is zero. That is, storing in a memory in advance data Y0 produced when a voltage corresponding to the point γ is inputted to a first pulse circulation circuit, and assuming that data produced when the analog input voltage is inputted to the first pulse circulation circuit is Y, and that data produced when a reference voltage Vref is inputted to the second pulse circulation circuit is Yref, then, the result of compensating operation (Y−Y0)/(Yref−Y0) is defined as the A/D conversion data.
In an A/D converter circuit disclosed by patent document 3, a voltage that an analog input voltage is added to an offset voltage and a voltage that the analog input voltage is subtracted from the offset voltage are inputted to a pulse delaying circuit, respectively, and a difference of the circulation numbers for these voltages is defined as A/D conversion data. Accordingly, relation between the analog input voltage and the A/D conversion data is linearized. In this case, by preparing A/D conversion data of a reference voltage as reference data, the temperature compensation is performed by dividing the produced A/D conversion data by the reference data.
As described above, in the temperature compensation technology to TAD in the past, the range of analog input voltage, which can be compensated to a temperature change, is dominated by the high voltage side or the low voltage side of a voltage range (for example, 0 to 5V) of the power supply voltage for operation usually used in a sensor device etc. Therefore, sufficient temperature compensation was not attained for the central voltage (for example, a voltage range centering on 2.5V), which is most used as an output voltage of a sensor etc. The range of analog input voltage, for which the temperature compensation could be performed successfully, was also narrow.
Since a digital operation including a division was necessary, the operation required time and shortening of conversion time was difficult. In a method using the point γ, at which the difference of temperature characteristics is zero, it is necessary to perform actual measurement of the temperature characteristics for each product in an inspection process after manufacture. Accordingly, production cost becomes high.